1. Field of the Invention
This invention relates to an improved digital signal processing method and apparatus for demultiplexing and multiplexing frequency division multiplexed channels and more particularly, to a lightweight, compact, low power system which synthesizes a polyphase filter, fast Fourier transform architecture suitable for use in applications with a large number of channels.
2. Related Art
In the prior art, digital polyphase filters (PPFs) are used in combination with fast Fourier transform (FFT) algorithms to efficiently demultiplex multiple, frequency division multiplexed (FDM) channels. Similarly, the polyphase filters are used in combination with inverse FFT algorithms to multiplex separate signals onto FDM channels. An N branch polyphase filter is realized by factoring (or partitioning) a desired prototype filter""s transfer function (or impulse response) into N sub-filters. When coupled with an N point FFT (or IFFT) as shown in FIGS. 1 and 2, an N branch PPF can be used to implement a bank of N bandpass, N:1 decimating (or 1:N interpolating) filters for demultiplexing (or multiplexing) up to N FDM channels. The MCD/M thus realized represents the most computationally efficient system known for both demultiplexing and multiplexing large numbers of FDM channels.
The minimum order necessary to synthesize the required polyphase prototype filter is dictated by the ratio of the guard frequency band (separating adjacent FDM channels) to channel center frequency spacing, filter passband gain uniformity, channel cross-talk attenuation requirements, and the number, N, of FDM channels to be either demultiplexed or multiplexed. Given a fixed guard band to channel spacing ratio and fixed filter response requirements, the minimum order of the polyphase prototype filter grows linearly with N.
Cellular, Personal Communication Systems (PCSs) and other emerging telecommunications applications continue to demand increasingly larger numbers of ever more closely packed FDM communications channels. Such systems (e.g., satellite based cellular systems) often place stringent constraints on the power, mass and physical dimensions of the digital MCD/M subsystem designed to generate the constituent FDM communications channels. These constraints necessitate the use of short sample word lengths (e.g., from 8 to 12 bits) and fixed point arithmetic in any practically realizable MCD/M system for the applications in question. Short sample word lengths and fixed point arithmetic are, however, inherently incompatible with accurate, high order prototype filter realizations. Indeed, in some applications, the indicated (higher order prototype) polyphase filter may not even be physically realizable within the finite word length and fixed point arithmetic dimensions necessary to meet the MCD/M subsystem""s power, mass and size constraints.
An object of this invention is the provision of a computationally efficient digital signal processing system, which synthesizes a polyphase filter-fast Fourier transform or a polyphase filter-FFT and inverse fast Fourier transform combination to respectively demultiplex or multiplex a large number of closely spaced FDM channels. That is, the provision of such a digital signal processing system that can be implemented as a small size, lightweight device with low power consumption for multiplexing and demultiplexing a large number of closely spaced FDM channels.
Briefly, this invention contemplates the provision of a digital signal processing system for multiplexing/demultiplexing a large number of closely spaced FDM channels in which sub-band definition filtering divides the FDM channel band comprised of xe2x80x9cNxe2x80x9d channels into xe2x80x9cKxe2x80x9d sub-bands in order to reduce the dimension of the polyphase filter-fast Fourier transform structure required to complete the multiplexing/demultiplexing. This reduces the order of the required prototype filter by a factor proportional to K. The number of sub-bands K is chosen so that it is large enough to ensure the polyphase filter-fast Fourier transform structure for each sub-band is realizable within a finite word length, fixed point arithmetic implementation compatible with a low power consumption. To facilitate efficient sub-band definition filtering, the real basebanded composite signal is inputted at a spectral offset from DC equal to one half the aggregate FDM channel bandwidth for the N channels and the signal is sampled at a frequency 100% greater than the applicable Nyquist rate. The half band spectral offset and oversampling by 100% above the theoretical Nyquist rate facilitate the use of computationally efficient bandshift and symmetric half-band Finite Impulse Response (FIR) filtering.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digits in the corresponding reference number.